Best configuration for a Hobby CNC

 In another thread Clive Foster Wrote:

Thinking out of the box.

If you only want to make a few varieties of parts like those in your pictures I wonder if one of the more rigid varieties of small desktop CNC router devices might be up to the job. There seem to be a number of versions built in Plano-Mill style with the side arms fixed and the table moving in both X and Y which should be much more rigid than the original versions where the side arms move.

Working areas look to be maybe 9" square by 3" deep. Which might be enough. Obviously not up to heavy, proper engineering level, cuts but a carbide cutter whizzing round at high speed will get the job done, eventually. Especially if you saw out most of the waste first. Hardly matters if it takes 3 or 4 hours to do something you could do in half an hour on a ME size bench mill as you don't have to stand over it. Light alloy should be well within the capabilities of a decent one. Steel and stainless steel ought to be possible with a well made one.

But does anyone know for sure?

I'm actually a little surprised that no one has yet marketed a small CNC Plano-Mill style machine with a head of basically same style as the small bench top mills. Basically the subtractive machining equivalent of the desktop 3D printers with similar, but a bit larger, work envelope to be used in the same manner with small carbide cutters running under High Speed Machining style low load strategies. Which would probably be just what you want.

Folk on this forum are model engineers who take great pleasure in using their machines and building up their skills but there is a fair bit to learn and fair amount of kit to even get properly started so you can turn out good work of something move than very basic complexity. Like those in your pictures.

The water slide down the rabbit hole is endless. Fun though. If thats what you want to do. OK I do 12 inch to the foot work, 45 years in over £25,000 spent on "stuff" adn mabe £250,000 to start over all new.

Clive

Reply

I'm not sure why you feel the need for a traditional mill type head on a gantry style machine Clive? Buy the very nature of the small tools being used a high speed spindle that can take the tool directly is far better than a belt or geared head in the smaller sizes and there is no real need for a quill. The budget end machines will just have a small ER spindle built in but as you get a bit larger (expensive) you can get some very nice powered spindles with quick change INT20 or 25 nose ready plumbed for air drawbar and the larger ones plumbed for water cooling. Even some of the ones that use a router type spindle have things like a small lever for quick tool changing. Most of what I have seen the gantry stuff doing seems to show non ferrous but as you say with light enough cuts steels should be OK. Not forgetting the need to be able to draw a part in CAD first and knowing what to tell the CAM you want in the way of feeds and speeds. Plus the cost of CAD & CAM software if not just for hobby use.

Edited By JasonB on 28/08/2020 13:02:41

Edited By JasonB on 28/08/2020 13:03:56

Jason

Drifting off topic a bit here.

Its not a case of actually feeling the need for a traditional mill head on a gantry style machine more a case of considering what is reasonably possible at an affordable price with a (potentially) viable target market. Given the absence of HSM machining strategies with current model engineer level CNC controls something more rigid than a router style head is needed to handle conventional cuts. Mix and match from components already in production is also a lot more economical than design from scratch. Especially when you aren't sure which way the market will develop. An alternative to the conventional ME small CNC machine based on manual mill components.

The uprated, fixed gantry, router style machine has to be considered as the subtractive machining equivalent to a 3D printer and would be used in the same way. Control strategy inherently HSM, probably much closer to 3D printing running "backwards" than conventional CNC. Reasonable CAD has to be taken as a given for folk starting out to do own design components now.

It will be interesting to see where the whole MakerSpace thing goes in the next few years as its now, in principle, possible to go straight to CNC subtractive machining without learning to drive manual machines first. But the equipment doesn't really seem out there as a coherent yet. Waiting for the Rep-Rap equivalent methinks.

But maybe someone knows differently which is why I started this hare running.

Hobby / ME affordable technology generally seem to run about 20-30 years behind industry and straight to CNC has been a norm for a fair few years now.

Clive

Edited By JasonB on 28/08/2020 13:04:43

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I certainly use the high speed or adaptive strategies even if I tone down the speed a little to suit my machine and generate them with F360 which Mach3 is happy to run. I think even the small hobby 3020 routers will do it.

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Clive, maybe you are talking about something like a rigid VMC where a basic L shape would support the milling table on the bottom leg and the uprighy part support the spindle. Both running on simple slides for the basic "reprap" machine and upto linea rails for a more serious machine. This would certainly be mored rigid than a lighter gantry style but I should think most users would want to take advantage of that rigidity to allow for more aggresive and faster HSM cuts than traditional.

There is also the fact that a gantry router can be assembled from simple parts, there are even ones about that use 3D printed fittings where as the rigid L shaped frame of a VMC type machine would need casting or welded fabrication. Home resin casting is certaily an option which some good examples about and I'm sure there are a few who can weld better than me that have gone down that route. Even seen a very rigid gantry one built from Granite bonded together with JBWeld that could shift aluminium at a fantastic rate. PM me for a link if anyone wants to see the video.

These Societe Genevoise jig borers did good work for many years so gantry style is not a new format, but they weren't designed to remove huge amounts of material quickly, more about accuracy, so the duty can be likened to a cnc 3D mill but that's a heavy old table to keep moving around at a high feedrate.

Emgee

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Good video here outlines the advantages of this kind of toolpath.

Thanks for the link to the video Blowlamp, I'd not heard of this system before.

Brian

Thanks for the move Jason. That hare took off a lot faster and further than I intended! I just wanted to "look at things from the other side of the fence". Which can be very useful tactic when discussions start to bog down in "I think I'd like, maybe, well I'm not sure (and how the heck can I afford it)" land.

Barrie

By straight to CNC I meant trained machinists going straight to CNC machines and learning how to exploit them rather than learning to cut metal on manual machines first. (The perils of editing and not being able to proof read what you actually wrote!)

I knew that Fusion was growing HSM strategies but I'd always thought that conventional hobby level machines wouldn't have the speed to exploit them. Especially as they are G-Code based and I have my reservations about G-Code (see below).

Jason

Yep, pretty much my thoughts but there is a historical background.

In 2004 the MoD made me redundant from my job as a Scientist / R&D Engineer / Proof of Concepts guy so I was looking for something to preserve my money and tide me over until I had to go into full time carer / house person mode. Back in the day I'd built a multi-axis motion system for evaluating certain aspects of optical performance for a rather hair-brained idea that worked at lab scale but .... . This thing was basically a 3 (or more) D vector path following drive which I reckoned could easily have been made to run a (small) milling machine at constant tooth load and constant cut velocity. Not complicated to implement once you have the Eureka moment!

I figured that commercial exploitation might be possible using a benchtop plano-mill machine something like the modern, kit built, fixed side router but better engineered and more sturdy. Using things like the extruded X format alloy rails used in optical lab gear for the cross bar and VMC style head with linear rails under the table et al. A proper cast iron machine would have been completely unaffordable but, although still not cheap back then, a build up of that style could have been viable. Stiffness should have been sufficient for a work volume up to maybe 2 ft x 18" x 1 ft at the low cutter loads I envisaged. Cutter costs then were one of my bigger worries as to the viability of the whole thing.

I bought a Taig CNC machine to test out the proposed commercial application. Took me about an hour to conclude that the folks who implemented 3D G code were smoking whacky baccy. Also concluded that Mach is both an amazing tour de force of programming and completely the wrong way to go about driving a small mill from a standard PC. I know why G code is the way it is but I learned my control strategies on guided weapons so naturally think in terms of curves of pursuit, error reduction and vectors not point to point. I also discovered that some of the computer programs I was going to need didn't exist and never would exist as the way thing's were done was different to what I expected.

Then MoD said come back as a consultant for £££ lots so I dropped the whole thing.

Fast forward to 2020 and thanks to smartphones and 3D printing all the extra computer programming bits I'd need exist. Albeit in rather different form. The mechanics are cheaper. The engineering remains viable for a limited range of bench top sizes and carbide cutters are affordable. If I were 10 years younger I'd give it a proper go but, at 66 I've got to ration the good years I have left.

Clive

PS I initially learned programming on a hybrid digital-analogue computer!

Barrie

That picture of a prosumer CNC is interesting. Somewhat similar to my 2004-2006 thoughts but grossly under engineered.

Clive

Edited By Clive Foster on 28/08/2020 18:49:29

Clive;

FYI: About a decade ago, at SIGGRAPH, a 3d gantry-style router was presented, called "DIYLILCNC", and plans were available by download.

It used a Dremel for the spindle, and could do aluminum. This was when 3D printing was really doing well with home-built machines; a group thought that "subtractive machining" was something that might fly with those without tons of money for the then-current CNC mills. It used LinuxCNC if I remember correctly, so was certainly within the price range of the youngsters.

The guys seemed to be there for a couple of years, but faded out.

Maybe they were before their time, or maybe the Chinese routers are where it's at.

John.

I'm feeling my age : HSM = Home Shop Machinist

Prosumer Eh?

At least Blowlamp's video was sort of in English.

That's because high speed is a misnomer. It's not necessarily about higher spindle speeds or feedrates. What they try and do is keep the cutter engagement constant so feeds and speeds can be maximised. The outcome is the the overall machining time is reduced - hence high speed. In contrast a simple 2D path might use a given stepover, but depending upon the geometry may decide to do a full width cut over a short period. In that case the speeds and feeds (and DOC) are determined by the full width cut, not the majority of the machining. So overall time might be slower. Another bonus of constant engagement is less chance of chatter due to short term large engagements.

Andrew

HSM = High Speed Machining.

John

DIYLILCNC still seems to be around but it's very low end "look what I made" device. Current version seems to be made, mostly, of plywood. I've no doubt that in careful hands it can do surprisingly good work.

But a long step from a commercially viable offering.

As I found out hitting a sensible price / performance ratio for something novel that folk are willing to shell out lots of beer tokens for to do useful work and earn a wage from is hard. Especially if production is small workshop based with bought in components rather than having factory resources behind it.

I have a sneaky feeling that even if I'd got mine up and running the spindle costs would probably have tipped it over the viable price / performance ratio. My target was 3 (or at most 4) times the price of a CNC Taig with significantly larger work envelope. Beyond that the whole commerciaisation thing wasn't going to fly. Big difference between a "credit card" cost range machine and one needing a proper Cap-Ex process however informal.

Clive

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